There are a number of well known non-invasive imaging techniques for the diagnosis and treatment of patients. These techniques generally allow a physician to obtain high fidelity views of the human anatomic structure without the advent of invasive procedures. Such imaging systems which provide a cross-sectional (tomographic) view of the human body include computer-aided tomography (CAT scans) x-ray imagers, magnetic resonance (MRI) imagers, positron emission tomographic scanning (PET), and magnetoencephotographic scanning (MEG). Typically, when a patient is scanned using one of these conventional cross-sectional imaging techniques, the patient's entire body or limb is imaged or mapped and the resulting anatomic topology is stored in an imaging database for later use.
While CAT scans, MRI's, PET's, and MEG's allow imaging of the entire body or entire limb requiring treatment, ultrasonic hand-held transducers are generally used to obtain a more localized image of a treatment area. In general, the hand-held transducer contains an imaging element which includes an ultrasonic sensor for placing in contact with the patient's skin. During operation, the imaging element of the transducer sends ultrasonic waves from the transducer into the patient's body to the region of interest, and scattered waves reflected from the region of interest within the body are collected by an ultrasonic sensor within the transducer. The resulting electrical signals can then be processed to produce an image corresponding to the structure imaged by the transducer.
Many imaging systems, such as, those described above, include a feature that allows the operator to indicate the source or location of the scanned image. In particular, the operator makes use of a model pattern representing the human body. In marking the location of the recorded image, the operator places a specially configured cursor representing the image plane of the scanning device over a model pattern to illustrate the imaging plane's orientation with respect to the patient's body.
For hand-held transducers, however, any movement of the transducer's orientation with respect to the patient's body changes the orientation of the imaging plane. This change in the imaging plane thereby requires the operator to readjust the specially configured cursor with respect to the model pattern. As a consequence, if the patient's body shifts between taking images, or if the transducer must be repositioned after imaging, it is often difficult to recapture the prior image location.
Moreover, an additional problem associated with the aforementioned scanning techniques is that each imaging process is particular to a patient, and thus sensitive to the patient's position with respect to the imaging device. Therefore, each set of images has a discrete, unique orientation related to a single patient resulting from a single scanning session. Because of their unique or distinct features, images formed at different times, or from different vantage points, cannot be suitably compared on a point-by-point basis. This prevents an accurate comparison of the scanning regions from one scanning session to another, and from one patient to another. Such a comparison would be desirable in cases where the practitioner wishes to accurately compare scanned images from a healthy patient to the images of the patient undergoing the treatment, or where a practitioner wants to assess the responsiveness or improvements of a patient to a particular treatment. When such a comparison is desired, a system which enables the practitioner to place a transducer or other medical device in a prior imaging position would be tremendously beneficial.
Prior art systems for aiding or guiding the positioning of medical devices are generally cumbersome and complex. For example, U.S. Pat. No. 5,748,767 issued to Raab discloses a method for aiding a medical practitioner in the re-positioning of a surgical tool in a prior location, such as when the patient has moved. This re-positioning of the apparatus is accomplished by providing a correlation between the coordinates of the pre-treatment image and the image acquired during the treatment procedure. In addition, Raab necessarily requires the use of an apparatus which can transpose the imaging information from the reference system of the imaging system to the reference system of the apparatus. The system disclosed in Raab provides a method to enable the surgical tool to be positioned and re-positioned in relatively the same position with an acceptable accuracy by coordinating the pre-treatment and treatment reference systems.
During operation of the Raab system, the pre-treatment and treatment coordinates are continually calculated with respect to a specially designed reference block attached to an electrogoniometer. The electrogoniometer is further used to determine the orientation of the instruments used in the treatment process. This system, however, uses a mechanical linkage for maintaining the surgical tool in a fixed relationship with the reference block. Such machinations of the probing process creates a system that is relatively cumbersome for the practitioner using a hand-held transducer.
An additional problem with existing systems is found in the difficulty with which a prior imaging plane can be recaptured for comparison purposes, a problem which becomes even more significant with the use of hand-held transducer. For example, in one therapeutic application using a hand-held transducer, the objective of the treatment is to create a very well-placed thermal gradient in the treatment area to selectively destroy certain regions thereof. An example of this is the hypothermia technique wherein a temperature near about 43 degrees Celsius is required to be maintained in the specific treatment area, or the focused ultrasound surgery technique which has a goal of elevating the treatment area temperature to above and beyond 55 degrees Celsius. During the therapeutic treatment process, the physiological response of the target tissue is directly related to the spatial extent and temporal duration of the heating pattern. Consequently, in order to appropriately perform feedback and control of the therapeutic treatment process, it is absolutely essential to control the temperature in the target tissue so as to know whether or not the temperature in the treatment region has been raised to a level that produces a desired therapeutic effect or the destruction of the tissue. Moreover, as with the hypothermia and focused ultrasound surgery techniques, and any other technique whereby the success of the therapy must be evaluated from one treatment session to the next, it is critical to be able to accurately image the treatment area undergoing treatment for comparison to subsequent treatment sessions.
Another method for enabling a particular image to be recaptured from one session to the next may be accomplished by using a three-dimensional coordinate system that is fixed within a human anatomy. For example, U.S. Pat. No. 5,230,338 issued to Allen et al. discloses a method for interactively guiding a surgical tool, wherein three or more fiducial implants are implanted in three separate, spaced locations within the human body. With the Allen method, the three fiducial implants are arranged in a non-collinear manner such that a plane is formed which defines a three dimensional coordinate system. Once the external coordinate system is established, a scan of the treatment area is performed. During subsequent scans, the patient's orientation may change relative to the imaging apparatus, but the new orientation of the patient can be measured by locating the fiducial implants in relation to the imaging apparatus. In this manner, the images of subsequent scans of a patient relating to a new position or orientation can be configured to correspond to the earlier recorded scans for comparison. A disadvantage inherent in the Allen system, however, is that the fiducial implants remain implanted in the patient between imaging session. In addition, the Allen system does not facilitate the scanning or diagnosis of other patients, but only operates effectively for the same patient.
As described above, several prior art techniques exist for monitoring and recording the position of tools used in the diagnosis and treatment of patients, or for recapturing an image scanned in a prior scanning session. Generally, these techniques are complex as in the U.S. Pat. No. 5,748,767 issued to Raab, or are limited in their use, as in U.S. Pat. No. 5,230,338 issued to Allen et al. That is, use of such techniques essentially requires the implementation of cumbersome equipment, such as an electrogoniometer and reference block, or requires that elements of the system remain with the patient between scans, such as the implants in Allen. In that regard, such methods are inadequate for medical specialist who desire a simpler and less cumbersome manner to record a tool position or orientation which correlates with a prior image, to facilitate the return to the precise location of the prior treatment in that patient or, alternatively, a second patient undergoing similar treatment.
Thus, a need exists for a less cumbersome and easy to use system capable of monitoring and recording the position of a transducer to facilitate the desired-positioning of the transducer within a treatment area.